1. Field of the Invention
The present invention relates to a fixing apparatus used in an image forming apparatus such as an electro-photographic copying machine or printer.
2. Description of the Related Art
An external heating type fixing apparatus is known as a fixing apparatus used in an electro-photographic copying machine or printer. The external heating type fixing apparatus typically includes a fixing roller, a rotator for heating the fixing roller, and a backup member that is in contact with the fixing roller to form a nip portion. As the rotator for heating the fixing roller, a cylindrical film with which a heater is brought into contact, and a roller having a halogen heater therein can be used, for example. A recording material bearing an unfixed toner image is heated at the nip portion while being conveyed, and the unfixed toner image on the recording material is heat-fixed to the recording material.
In the external heating type fixing apparatus, offset toner adhering to the fixing roller may move and adhere to the rotator for heating the fixing roller. Because the rotator does not make contact with the recording material or does not have a cleaning unit, the toner tends to accumulate on the rotator. When the accumulated toner becomes a large lump, the lump may fall on the recording material being conveyed at the nip portion, causing a defective image.
Japanese Patent Application Laid-Open No. 3-25481 discusses a fixing apparatus in which the toner releasing property of a rotator is made higher than the toner releasing property of a fixing roller to prevent the offset toner on the surface of the fixing roller from adhering to the rotator for heating the fixing roller. In this fixing apparatus, the adhesion of the offset toner to the fixing roller is stronger than the adhesion of the offset toner to the rotator, and consequently, the offset toner on the fixing roller is more likely to accumulate on the surface of the fixing roller without adhering to the rotator. This prevents the offset toner from moving from the fixing roller to the rotator for heating the fixing roller.
However, in a heat-fixing operation for a recording material, paper dust such as paper fibers and loading materials (fillers) formed of inorganic substances such as calcium carbonate and talc contained in the recording material may adhere to the fixing roller although the adhesion amount is very small. Since the toner contained in contamination, which is a mixture of such paper dust and the offset toner moved to the fixing roller, has weak adhesion, the movement of the contamination from the fixing roller to the rotator may not be sufficiently suppressed only by the configuration discussed in Japanese Patent Application Laid-Open No. 3-25481.